(a) Field of the Invention:
The present invention relates to an apparatus for measuring reflectivity.
(b) Description of the prior art:
Anti-reflection coating technique is an important art indispensable for various kinds of optical systems. Without the application of this specific technique, there arises the inconvenience that lens systems such as zoom lens and objective lens of a high-grade microscope which are comprised of a number of component lens would develop a substantial drop of light-transmittance, and such lens systems are never acceptable for practical use. In order to confirm that the lens system is provided with a proper anti-reflection coating, it is necessary to conduct an actual measurement of the reflectivity of the anti-reflection film formed on the lens surface. Although reflectivity measuring devices have been known since a long time ago, devices of this kind, in general, allow the measurement of reflectivity of only a planar surface, so that it has been the usual practice to use, as a test-piece, a planar glass plate having the formation of an anti-reflective film on the surface thereof which is produced thereon under the same condition as would be formed on the surface of a lens, and to measure the reflectivity of the test-piece. With such a manner of measurement employed in the past, the actual reflectivity of the anti-reflection film formed on a lens surface is not known. The reflection light which comes from the rear surface of the test-piece is mingled with the measurement light so that the prior art measuring devices had the drawback represented by a low degree of precision of measurement.
Therefore, the present inventors have disclosed, in Japanese Patent Preliminary Publication No. Sho 54-133180, a reflectivity measuring apparatus which allows the measurement of the reflectivity of a curved surface and which prevents the intrusion of the reflection light coming from the rear surface of a lens. This apparatus has such a construction as shown in FIG. 1. That is, in FIG. 1, reference numerals 1 and 2 represent two objective lenses which are disposed in such a way that their optical axes 3 and 4 cross each other at right angles. At the focal positions of these two lenses, there are disposed a reference surface 5 and a surface 6 for examination, respectively, which will hereunder be called simply an examination surface 6. In addition, a reflecting mirror 7 having reflecting surfaces on both faces thereof is provided off-axially, i.e. outside of the optical axes, of these lenses 5 and 6. And, among the two monochromatic light beams which are incident to the lens 1 in parallel with the optical axis 3 thereof, the beam 8 is reflected at the rear surface of the reflecting mirror 7, and is collected by the objective lens 2 to enter obliquely onto the examination surface 6, and the light reflected at the examination surface 6 passes through the objective lens 2 and advances in parallel with the optical axis 4, and via a light-collecting lens 10 which is disposed coaxially with the objective lens 2, it is collected onto a light-receiving device 12 after passing through a stop 11. The light beam 9, on the other hand, is collected onto the reference surface 5 by the objective lens 1, and the reflection light coming from the reference surface 5 passes through the objective lens 1 and advances in parallel with the optical axis 3, and after being reflected at the reflecting mirror 7, it impinges onto the light-receiving device 12 in the same manner as does the light beam 8.
With this arrangement mentioned above, the measurement beam of light is squeezed into a fine spot light at the examination surface 6 and also at the reference surface 5, so that the reflectivity at a planar surface as well as at a curved surface can both be measured. Also, because the measurement light beam enters obliquely, it will be noted from FIG. 2 that the surface reflection light and the rear surface reflection light follow different optical paths relative to each other. Therefore, it is possible to eliminate the rear surface reflection light by the stop 11 which is provided in front of the light-receiving device 12.
According to the above-mentioned prior art example, however, there have been encountered the following problems, which are:
(a) because two objective lenses are employed, the optical system becomes complicated in construction and tends to have a large size; and
(b) because of the employment of a fine beam of light which is incident off-axially of the objective lens, there arises an insufficiency of the intensity of the measurement light, and accordingly it is affected by noises, and the degree of precision of measurement has tended to become degraded.